This invention relates to a blue light sensitive holographic recording article that could be used either with holographic storage systems or as components such as optical filters or beam steerers. In particular, the invention relates to the photosensitization of high performance holographic recording media to blue laser wavelengths.
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, so-called page-wise memory systems, in particular, a holographic system, have been suggested as alternatives to conventional memory devices.
A hologram stores data in three dimensions and reads an entire page of data at one time, i.e., page-wise, which is unlike an optical CD disk that stores data in two dimensions and reads a track at a time. Page-wise systems involve the storage and readout of an entire two-dimensional representation, e.g., a page, of data. Typically, recording light passes through a two-dimensional array of dark and transparent areas representing data, and the holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index imprinted into a storage medium. Holographic systems are discussed generally in D. Psaltis et al., xe2x80x9cHolographic Memories,xe2x80x9d Scientific American, November 1995, the disclosure of which is hereby incorporated by reference. One method of holographic storage is phase correlation multiplex holography, which is described in U.S. Pat. No. 5,719,691 issued Feb. 17, 1998, the disclosure of which is hereby incorporated by reference.
The advantages of recording a hologram are high density (storage of hundreds of billions of bytes of data), high speed (transfer rate of a billion or more bits per second) and ability to select a randomly chosen data element in 100 microseconds or less. These advantages arise from three-dimensional recording and from simultaneous readout of an entire page of data at one time.
A hologram is a pattern, also known as a grating, which is formed when two laser beams interfere with each other in a light-sensitive material (LSM) whose optical properties are altered by the intersecting beams. One choice of a LSM is a photosensitive polymer film. See, e.g., W. K. Smothers et al., xe2x80x9cPhotopolymers for Holography,xe2x80x9d SPIE OE/Laser Conference, 1212-03, Los Angeles, Calif., 1990. The material described in this article contains a photoimageable system containing a liquid monomer material (a photoactive monomer) and a photoinitiator (which promotes the polymerization of the monomer upon exposure to light), where the photoimageable system is in an organic polymer host matrix that is substantially inert to the exposure light. During writing of information into the material (by passing recording light through an array representing data), the monomer polymerizes in the exposed regions.
U.S. Pat. No. 6,103,454 and application Ser. No. 09/046,822, the disclosures of which are hereby incorporated by reference, also relate to a photoimageable system in an organic polymer matrix. In particular, the application discloses a recording medium formed by polymerizing matrix material in situ from a fluid mixture of organic oligomer matrix precursor and a photoimageable system.
The conventional photoimageable systems have generally been optimized for sensitization with green laser light having a wavelength of 532 nm. However, holographic recording using green laser is inherently limited by its wavelength. In general, a denser data-storage is possible using a shorter wavelength laser. Blue wavelength lasers have wavelengths of 400-490 nm (particularly 405 nm). Therefore, a photoimageable system for holographic recording that could be used with a blue laser is desired.
This invention in high performance holographic recording articles is based on novel holographic formulations that can be used with blue lasers, which provide several advantages to the holographic performance of these media. First, a denser data-storage is achieved due to the shorter wavelength of blue light as compared to a green light. Second, a higher dynamic range (M/#) is achieved in a given matrix/photopolymer formulation owing to a somewhat higher refractive index contrast inherent to the media components in the blue portion of the electromagnetic spectrum compared to the green portion. Third, the sensitivity of the blue sensitized media is higher as compared to the green sensitized media, hence affording more rapid hologram writing times.
In addition to these advantages, the blue sensitized media uses an entirely organic photosensitizer. The green photosensitized media, on the other hand, owe their sensitivity to a transition metal complex photosensitizer that is reduced during hologram recording. This reduced complex absorbs visible light, coloring the media, due to excitation of a d orbital electron to a d* state. Because this absorption interferes with the process of writing and reading holographic data, an oxidizing agent (e.g., tert-butyl hydroperoxide) is typically added to the green media to increase the oxidation state of the metal center by removing the d electron and its associated transition. In contrast, the blue sensitized media does not contain such a transition metal complex and therefore bleaches to completely transparent state at all visible wavelengths without any oxidizing or other additives. It is also noted that oxidizing agents tend to adversely impact thermal stability of the media and/or media components.
While affording these advantages, blue sensitization affords no apparent disadvantages to the optical quality of these media compared to green sensitization. For example, shrinkage levels are the same for sensitization with either color. Also, hologram quality, as exemplified by a close experimental match to Kogelnik""s coupled wave theory for volume holography, remains high.
Additional advantages of this invention would become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiments of this invention are shown and described, simply by way of illustration of the best mode contemplated for carrying out this invention. As would be realized, this invention is capable of other and different embodiments and its details are capable of modifications in various obvious respects, all without departing from this invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive.